The present invention relates to an image display apparatus and, more particularly, to a head- or face-mounted image display apparatus that can be retained on the observer's head or face.
A conventional image display apparatus is disclosed in Japanese Patent Application Unexamined Publication (KOKAI) No. 3-101709 (1991). FIG. 10(a) shows the entire optical system of the known image display apparatus, and FIG. 10(b) shows a part of an ocular optical system used in the image display apparatus. As illustrated in these figures, in the conventional image display apparatus, an image that is displayed by an image display device 20 is transmitted as an aerial image 25 by a relay optical system 21 including a positive lens, and the aerial image is projected into an observer's eyeball 24 (pupil) as an enlarged image by an ocular optical system 23 formed from a concave reflecting mirror 22.
U.S. Pat. No. 4,669,810 discloses another type of conventional image display apparatus. In this apparatus, as shown in FIG. 11, an image of a CRT 30 is transmitted through a relay optical system 31 to form an intermediate image 32, and the image is projected into an observer's eye by a combination of a reflection holographic element 33 and a combiner 34 having a hologram surface.
Japanese Patent Application Unexamined Publication (KOKAI) No. 62-214782 (1987) discloses another type of conventional image display apparatus. As shown in FIG. 12, the conventional image display apparatus is designed to enable an image of an image display device 40 to be observed directly by an observer's eyeball 42 (pupil) as an enlarged image through an ocular lens 41.
U.S. Pat. No. 4,026,641 discloses another type of conventional image display apparatus. In the conventional image display apparatus, as shown in FIG. 13, an image of an image display device 50 is transferred to a curved object surface 52 by an image transfer device 51, and the image transferred to the object surface 52 is projected in the air by a toric reflector 53.
U.S. Reissued Pat. No. 27,356 discloses another type of conventional image display apparatus. As shown in FIG. 14, the apparatus is arranged such that an ocular optical system 64 comprising a semitransparent concave mirror 60 and a semitransparent plane mirror 61 projects an object surface 62 onto an exit pupil 63.
Other known image display apparatuses include those which are disclosed in U.S. Pat. Nos. 4,322,135 and 4,969,724, European Patent No. 0,583,116A2, and Japanese Patent Application Unexamined Publication (KOKAI) Nos. 7-333551 (1995) and 8-234137 (1996).
In some of these conventional techniques, a reflecting surface and a transmitting surface, which constitute an optical system, are formed by using surfaces having a simple surface configuration with a strong symmetry, such as a spherical surface, a rotationally symmetric aspherical surface or a toric surface. Therefore, ray aberration and distortion produced by a decentered surface having a power have heretofore been impossible to correct simultaneously and favorably. Accordingly, the observer can view an image for observation only in a distorted condition and may feel discomforted. This may also make the observer's eye tired and cause him or her to have a headache.
In a case where the optical system is provided for each of the user's left and right eyes, another problem arises in addition to the above-described problem. That is, images viewed with the left and right eyes are distorted to be asymmetric with each other and cannot properly be fused into a single image. This problem becomes particularly remarkable when the performance of the image display apparatus is improved in order to enhance its dynamic presence by using a display device of high resolution, an ocular optical system of wide field angle, etc. Consequently, the functions of these high-grade devices are degraded. Moreover, in the case of displaying a figure, for example, the displayed figure appears to be distorted, making it impossible to correctly recognize the shape of the displayed figure and thus causing the dynamic presence to be completely lost.
Some of the above-described conventional techniques propose using surfaces having a complicated surface configuration with a little symmetry, such as an anamorphic surface having only two planes of symmetry and a plane-symmetry free-form (three-dimensional) surface having only one plane of symmetry, as a surface for simultaneously correcting such aberrations and distortion produced by a decentered surface. It is certain that these surfaces can correct aberrations due to decentration which cannot satisfactorily be corrected by an optical system using a spherical surface, a rotationally symmetric aspherical surface, or a toric surface. In the conventional techniques using such an anamorphic surface or a plane-symmetry free-form surface, however, the complicated surface configurations have not thoroughly been examined, and the proposed surface configurations are inadequate to correct aberrations due to decentration simultaneously and favorably. In particular, when the field angle is enlarged, the aberration correcting performance is deteriorated to a considerable extent. There have heretofore been no satisfactory specific surface configurations of anamorphic surfaces or plane-symmetry free-form surfaces capable of satisfactorily correcting aberrations due to decentration.